The major viral antigen of bovine leukemia virus was purified and its molecular weight determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of the viral antigen was about 24,000 daltons. Preliminary radioimmunologic studies show that the major antigen of bovine leukemia virus does not share common antigenic cross-reacting determinants with known type-C and B RNA viruses.